We consider the adsorption of bovine serum albumin (BSA) on spherical polyelectrolyte brushes (SPB). The SPB consist of asolid polystyrene core of 100 nm diameter onto which linear polyelectrolyte chains [poly(acrylic acid), (PAA)] are grafted. The adsorption of BSA is studied at a pH of 6.1 at different concentrations of added salt and buffer. We observe strong adsorption of BSA onto the SPB despite the effect that the particles as weil as the dissolved BSA are charged negatively. The adsorption of BSA is strongest at low salt concentration and decreases drastically with increasing amounts of added salt. Virtually no adsorption takes place at salt concentration of 0.1 M. Moreover, the adsorbed protein can be washed out again by raising the ionic strength from low to high values. A major driving force for the adsorption is located at a lower pH within the brush at low ionic strength. Thus, the isoelectric point of the protein may be approached or even reached. In this case strong interaction between the SPB and the protein results. Moreover, the negative charge of the polyelectrolyte interacts with the patches of positive charges on the protein. In this way the protein becomes a multivalent counterion within the brush and monovalent counterions will be released. All results demonstrate that the SPB present a new elass of colloidal carrier partieIes whose interaction with proteins can be tuned in a well-defined fashion.

The thermodynamics and the driving forces of the adsorption of beta-lactoglobulin on spherical polyelectrolyte brushes (SPB) are investigated by isothermal titration calorimetry (ITC). The SPB consist of a polystyrene core onto which long chains of poly(styrene sulfonate) are grafted. Adsorption isotherms are obtained from measurements by ITC. The analysis by ITC shows clearly that the adsorption process is solely driven by entropy while DeltaH > 0. This finding is in accordance with the proposed mechanism of counterion release: Patches of positive charges on the surface of the proteins become multivalent counterions of the polyelectrolyte chains, thereby releasing the counterions of the protein and the polyelectrolyte. A simple statistical-mechanical model fully corroborates the proposed mechanism. The present analysis shows clearly the fundamental importance of counterion release for protein adsorption on charged interfaces and charged polymeric layers.

We investigate the enzymatic activity of glucoamylase and beta-glucosidase adsorbed on a novel type of colloidal particles. The particles used consist of a poly(styrene) core onto which long chains of poly(acrylic acid) or of poly(styrene sulfonic acid) are grafted ("spherical polyelectrolyte brush"). Proteins adsorb spontaneously onto these particles from aqueous solutions if the ionic strength is low. Moreover, the colloidal stability is not impeded by the adsorbed proteins despite the fact that up to 600 mg of enzyme is adsorbed per gram of the carrier particles. The activity of immobilized glucoamylase and beta-glucosidase adsorbed onto these particles is analyzed in terms of the Michaelis-Menten parameters. This analysis shows that both enzymes keep nearly their full activity. The Michaelis constant K(M) differs only slightly from the K(M) value of the native enzyme when the amount of adsorbed enzyme is raised despite the high local concentration of immobilized enzymes. All data demonstrate that spherical polyelectrolyte brushes present a novel way to immobilize enzymes.